The induction of a lymphopenic state frequently results in the development of organ-specific autoimmunity which is mediated by CD4+ T lymphocytes. Regulatory CD4+ T cells are selectively depleted by those procedures used to induce the lymphopenia. One implication of these findings is that autoreactive T cells which are capable of inducing autoimmune diseases exist in normal adult animals, but are maintained in a dormant or inactive state due to the suppressive functions of these regulatory T cells. We have demonstrated that the regulatory T cells can be easily identified in normal lymphoid tissues by co- expression of CD4 and the interleukin-2 receptor alpha chain (CD25). Transfer of CD4+ CD25- T cells to immunoincompetent mice results in the development of autoimmune disease which can be prevented by co-transfer of CD4+CD25+ cells. Our recent studies have focused on defining the mechanism of action of the CD4+CD25+ in vitro. CD4+CD25+ T cells are completely anergic to stimulation via their T cell receptor due to an inability to produce IL-2. When mixed with CD4+CD25- cells, they suppress proliferation by blocking transcription of the IL-2 gene in the CD25- population. The suppression was not cytokine mediated, required activation of the CD25+ T cells, and cell contact between suppressors and responders. The suppressor cells do not inhibit the generation of costimulatory molecules on antigen presenting cells, nor does excess costimulation abrogate suppression. Although the CD25+ population is heterogeneous with respect to expression of other membrane markers characteristic of memory/activated T cells, we have been unable to define a subpopulation of CD25+ cells which does not exhibit suppressor function. We have recently been able to isolate antigen-specific CD4+CD25+ T cells from mice which express a transgenic TCR on a conventional background. Upon activation by their target peptide, the CD25+ TCR transgenic cells can inhibit the responses of CD25- T cells to the same target antigen as well as the response of CD25- T cells specific for a distinct peptide. This indicates that once activated suppressor effector function is completely antigen-non- specific. One of the major problems is determining the specificity of the regulatory T cells is thatthe target antigen specificity of the autoimmune effector population is also unknown. We have demonstrated that post-day 3 thymectomy autoimmune gastritis in BALB/c mice is mediated by CD4+ T cells that recognize the H/K ATPase, the proton pump, of gastric parietal cells. We have further defined the fine specificity of the pathogenic H/K ATPase reactive population. The bulk population of T cells isolated from the draining gastric lymph node reacts with the alpha chain of the enzyme and only weakly with the beta chain. Two T cell clones were isolated and they react with distinct peptides from the alpha chain (amino acids 630-641 and 889-901, respectively). One clone secretes a Th1 pattern of cytokines, while the second clone has a Th2 phenotype. However, both clones induce autoimmune gastritis upon transfer to nu/nu mice; the capacity of the clones to transfer disease is blocked by co-transfer of CD25+ T cells from normal mice. Thus, the immunoregulatory CD4+CD25+ T cell population is able to inhibit the activity of highly stimulated T effector cells. We have isolated the TCR genes from one of the clones and produced TCR transgenic mice which express the receptor which recognizes the H/K ATPase. - Autoimmunity, regulatory T cells, gastritis, T cell clones